Annually resolved North Atlantic marine climate over the last millennium.

D J Reynolds,P G Butler,J D Scourse,C A Richardson,J Heinemeier,P R Halloran,K L Knudsen,A D Wanamaker,I R Hall,J Eiríksson,A J Nederbragt

doi:10.1038/ncomms13502

D J Reynolds, P G Butler + Show 9 more

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https://doi.org/10.1038/ncomms13502

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Abstract

Owing to the lack of absolutely dated oceanographic information before the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions (for example, Medieval Climate Anomaly-Little Ice Age, MCA-LIA). Here we present analyses of a millennial-length, annually resolved and absolutely dated marine δ18O archive. We interpret our record of oxygen isotope ratios from the shells of the long-lived marine bivalve Arctica islandica (δ18O-shell), from the North Icelandic shelf, in relation to seawater density variability and demonstrate that solar and volcanic forcing coupled with ocean circulation dynamics are key drivers of climate variability over the last millennium. During the pre-industrial period (AD 1000–1800) variability in the sub-polar North Atlantic leads changes in Northern Hemisphere surface air temperatures at multi-decadal timescales, indicating that North Atlantic Ocean dynamics played an active role in modulating the response of the atmosphere to solar and volcanic forcing.

Highlights

Owing to the lack of absolutely dated oceanographic information before the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions
The DR data represent the regional difference from the global mean ocean radiocarbon reservoir age, which on the North Icelandic shelf reflects the degree of entrainment of relatively old polar Arctic Intermediate Water (AIW) into the younger Subpolar Mode Water (SPMW) transported by the NIIC21,25
The coherence between the North Icelandic shelf DR21 and d18O-shell series indicates that during periods characterized by relatively older DR values, due to a relative increase in the proportion of AIW within the North Icelandic Irminger Current (NIIC), the d18O composition of the shell aragonite increases, indicative of an increase in the density of the ambient seawater bathing the site

Summary

Introduction

Owing to the lack of absolutely dated oceanographic information before the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions (for example, Medieval Climate Anomaly-Little Ice Age, MCA-LIA). The difficulty in constraining the role of coupled ocean–atmosphere mechanisms in climate variability to a large extent results from the limited length of oceanographic instrumental time series[7,8] and the low temporal resolution and dating uncertainties associated with marine sediment archives These issues hinder the identification of causal relationships through for example lead–lag analysis. Radiocarbon determinations from deep sea benthic foraminifera indicate changes in the deep-water composition of the Atlantic[24], coincident with these upper ocean shifts These studies demonstrate that variability across the North Atlantic marine environment was synchronous, within the temporal uncertainties, with the gradual reduction in Northern Hemisphere surface air temperature (NHSAT) over the MCA-LIA. The somewhat weaker (B0.3 Sv) inner NIIC retains its SPMW characteristics as it follows the inner North Icelandic shelf eastward at water depths of up to B100 m (Fig. 1c)[27]

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